Electronic packages which include a thin film circuit member as an integral part thereof are known in the industry, with examples being defined in U.S. Pat. Nos. 4,849,856 (J. Funari et al) and 4,914,551 (M. Anschel et al), both of which are assigned to the same assignee as the instant invention. As mentioned therein, a main objective in the design of such electronic packages is to provide these packages with as high current densities as possible within a minimum of space. Such miniaturization efforts, while providing several highly advantageous features, also present various engineering problems in both the manufacture and operation of these structures. In the aforementioned U.S. Pat. Nos. 4,849,856 and 4,914,551, for example, solutions to assure effective package heat removal during operation as well as facilitating package assembly are defined.
One particular problem facing designers of electronic packages containing thin film circuit structures is the essential requirement to provide sound electrical connections between the relatively large number of extremely small conductive elements (signal, ground and/or power conductors) and the respective contact locations (e.g., at the semiconductor chip and other circuit locations, such as on a printed circuit board or similar substrate) to which the chip is electrically coupled. As will be defined herein, the present invention defines both such a package and a method of forming such electrical connections in a film member and electronic device for use therein wherein relatively high circuit densities are attained. As will be further defined, such connections and the resulting structure are accomplished using solder. Even more particularly, such connections are achieved without use of chemical flux.
As is known, soldering represents a common technique for joining various objects together, including thin film circuits and other electronic devices. Typically, the conductive portions of these structures (usually metallic, e.g., copper) have been coated with a chemical flux prior to joining. Solid solder is then placed between the flux-coated objects or a flux-filled solder paste is screen-printed onto one of the conductive portions and heated to a molten state such that the molten solder coats the portions of these objects which in turn have been coated with such flux. Thereafter, the molten solder is cooled to form a joint between the two objects.
As is known, the flux serves to chemically reduce oxide which may be on the surface of the metallic objects being joined, and also on the surface of the molten solder. Such oxide prevents solder wetting between these surfaces, resulting in an ineffective bond. Using flux, this material volatilizes and is burned off during the bonding operation, causing certain undesirable effects such as gas generation which in turn can become incorporated within the molten solder. This may further result in voids in the bonding region, which in turn may weaken the final physical joint. Such voids further decrease the cross-sectional area of such a connection, which, if designed for providing electrical interconnection, may thus reduce the conductivity between the two conductors being joined. Still further, flux residue which is not totally volatilized may corrode the resulting solder joints as well as the objects being joined.
In microelectronic applications (e.g., for electronic package structures as defined herein), solder bonds are commonly used. In one technique, an electronic device (semiconductor chip) is mounted onto a packaging substrate (thin film structure) in a "flip-chip" orientation. In this orientation, the surface of the chip having contact locations is placed facing the thin film circuit member. The contact locations on the chip and the respective thin film conductive elements are electrically connected using solder structures, also referred to as controlled collapse chip connections (C4's).
As defined below, the present invention provides a significant improvement over currently known soldering techniques for performing electrical connections in miniaturized electronic packages. The method as defined herein assures sound, effective electrical connections between extremely small circuit elements. The method is relatively safe and can be adapted to mass production techniques, thus reducing the overall cost of the final package structure.
It is believed that an electronic package and method for forming electrical connections between the various elements for use in such a package possessing the advantageous features described herein would constitute a significant advancement in the art.